1. Field of the Invention
The present invention relates to a medicament dosing system and, in particular, to a medicament dosing system operating according to the overpressure principle.
2. Description of Prior Art
Lately, medicament dosage is carried out predominantly by dosing systems operating according to the overpressure principle. A schematic representation illustrating the over-pressure principle is shown in FIG. 8. Such systems consist of a fluid reservoir 10 and of a flow resistor 12 arranged e.g. on or in a fluid line 14 connected to the fluid reservoir 10. A pressure transmitter means 16 serves to apply pressure to the liquid medicament contained in the fluid reservoir 10. The pressure transmitter means produces a pressure p, whereby the fluid reservoir 10 is acted upon by a specific overpressure P.sub.1 relative to the pressure p.sub.0 at the outlet of the flow resistor 12. The pressure p.sub.1 substantially corresponds to the pressure p produced by the pressure transmitter means 16. When the system is in operation, a flow Q is caused by the differential pressure applied to the flow resistor 12.
For a circular cross-section of the flow resistor, the magnitude of the flow Q can be calculated according to the known Hagen-Poiseuille law: ##EQU1## The flow rate Q is determined by the following influencing quantities: the viscosity of the medium,
the effective flow cross-section .pi.R.sup.4 /8 and the length L of the flow resistor, PA1 the differential pressure .DELTA.p between the inlet and the outlet of the flow resistor, and PA1 the temperature as an indirect influencing quantity, e.g. by the temperature-dependent viscosity of the fluid. PA1 a fluid reservoir for receiving therein a pressurizable liquid medicament; PA1 a temperature sensor for detecting the temperature of said liquid medicament; PA1 a fluid channel which is provided with a flow resistor and which is in flow communication with the fluid reservoir; and PA1 a hose means which is connected to the fluid channel; and PA1 a squeezing valve means for squeezing the hose means together; and PA1 a control means which is coupled to the temperature sensor and the squeezing valve means so as to control a flow rate of the liquid medicament by clocked actuation of said squeezing valve means depending on the temperature detected. PA1 a fluid reservoir for receiving therein a pressurizable liquid medicament; PA1 a fluid channel which is provided with a flow resistor and which is in flow communication with the fluid reservoir; and PA1 a hose means which is connected to the fluid channel; and PA1 a temperature sensor for detecting the temperature of said liquid medicament; PA1 a squeezing valve means for squeezing the hose means together; and PA1 a control means which is coupled to the temperature sensor and the squeezing valve means so as to control a flow rate of the liquid medicament by clocked actuation of said squeezing valve means depending on the temperature detected.
For other flow cross-sections, analogous rules can be determined which differ from the rule mentioned in the above equation substantially with regard to the taking into account of the effective flow cross-section of the flow resistor. Such analogous rules e.g. for micromechanically produced flow resistors are described in "Micro Channels for Applications in Liquid Dosing and Flow Rate Measurement" M. Richter, P. Woias, D. Wei.beta., Proceedings of Euro Sensors X, Sep. 8 to 11, 1996, Leuven, Belgium, Vol. 4, pp. 1297 to 1300.
The technical embodiments of existing dosing systems vary greatly and use in a great variety of combinations mechanisms like mechanical systems, e.g. spring-pressure systems, electrochemical systems, e.g. electrolytic cells, thermopneumatic systems, e.g. the evaporation pressure of a highly volatile substance, and the gravitational force. The elements used as flow resistor are normally plastic capillaries, glass capillaries and metal capillaries.
According to the above equation, the radius R influences the flow rate Q through the term R.sup.4 to the fourth power, when the flow resistor is circular in cross-section. This means that, for achieving exact dosage, flow resistors having a high geometrical accuracy must be realized. Such an accuracy is only possible on the basis of a comparatively high technical expenditure. Simple systems including plastic capillaries are additionally disadvantageous insofar as the capillary stretches depending on the pressure applied, whereby the dosing accuracy will decrease.
In addition, comparable micromechanical embodiments used for glucose measurements by means of a microdialysis are known. In such a micromechanical embodiment, a microcapillary for flow adjustment is realized on a silicon chip together with glucose sensors. This known set-up is, however, not used for medicament dosage, but it uses the above-described over-pressure principle only for adjusting the flow rate of the carrier medium for the microdialysis.
A further known implementable micromechanical dosing system uses a solvent reservoir as a constant pressure transmitter and an array of micromechanically realized flow resistors for flow rate adjustment. The geometry of the whole flow path is varied by coupling and decoupling individual microflow resistors via respective microvalves associated therewith, whereby a dosing rate variation switched in steps is achieved. For measuring the flow, pressure sensors located at various points of the array are used. Such a system with an array of microflow restrictions does not permit a continuous adjustment of the dosing rate, the technical expenditure being, in addition, very high, since several microvalves and pressure sensors are necessary; hence, such a system can only be used in a limited field of application for reasons of costs.
The majority of the known dosing systems cannot be influenced from outside, i.e. the flow rate cannot be varied when the system is in operation, such varying being e.g. necessary for automatically observing a circadian rhythm when dosing a medicament. In addition, the influence of the temperature on the viscosity of the fluid and, consequently, on the dosing rate is normally not compensated in known dosing systems. Especially in the case of portable dosing systems, this may result in considerable dosing errors. When the flow rates to be adjusted are very low, .mu.l/min to pl/min, flow resistors with effective cross-sectional dimensions in the .mu.m range are required. Such cross-sectional dimensions cannot be produced by conventional techniques. Known dosing systems which do not use any complicated components as disposable components do not permit any continuous adjustment of the dosing rate.
DE-A-19501691 describes an non-invasive system for fluid flow supervision in which the infusion rate is adjusted by pulsed control of a fluid feed pump. The temperature of the fluid fed is supervised by a temperature sensor.
In DE-A-3827444 a method and a device for detecting a fluid flow in a line are described. The known device comprises a heating means attached to said line and two temperature sensors attached to said line upstream and downstream of said heating means. The temperatures measured at the temperature sensors serve to make statements on the existence of a flow, the direction of flow as well as the flow velocity.